We've recently added the ability to collect candidate OpenPGP keys from the outside world. (See bug 1667564.)

We could extend this to also use keys available in GnuPG - but only if our preferred access mechanism GPGME is configured and working.

In other words, at runtime, we can try to load GPGME (we already do in some configurations).
If GPGME works, then:

• automatically check for candidate recipient public keys, and allow the key assistant to import the public keys into Thunderbird's store (the work from bug 1627956)
• for public keys that we already have in Thunderbird's store, automatically check if we can obtain a newer version using GPGME.

Pushed by geoff@darktrojan.net:
https://hg.mozilla.org/comm-central/rev/655219ca9e61
Discover candidate OpenPGP keys from the GnuPG keyring, if external gnupg is enabled. r=mkmelin,rjl